MCQ Detailed View

When certain solid substances are exposed to the atmosphere, they interact with water vapor in the air. Some materials have the ability to absorb moisture directly from the air and eventually dissolve in the absorbed water to form a liquid solution. This process is called deliquescence. Substances that show this property are known as deliquescent substances. Common examples include calcium chloride (CaCl₂), sodium hydroxide (NaOH), and potassium hydroxide (KOH). These materials are highly hygroscopic and can absorb enough moisture to completely liquefy.

This property is important in physical chemistry because it relates to the equilibrium between solid and vapor phases. The ability of a substance to undergo deliquescence depends on the relative humidity of the surrounding atmosphere. When the humidity is higher than the critical relative humidity of the substance, deliquescence occurs, and the solid starts to absorb water vapor until it dissolves.

The other options describe different processes. Efflorescence is the opposite of deliquescence. In efflorescence, a hydrated crystalline substance loses water molecules to the atmosphere and becomes a powdery solid. For example, washing soda (Na₂CO₃·10H₂O) can lose its water of crystallization upon exposure to dry air. Dehydrogenation is a chemical reaction in which hydrogen atoms are removed from a molecule, commonly seen in organic chemistry and industrial processes like converting alkanes to alkenes. Desalination refers to the removal of salts from seawater or brackish water to make it suitable for drinking, which is unrelated to atmospheric moisture absorption.

Therefore, deliquescence is the correct process term when describing a solid that absorbs moisture from the air and turns into a solution. This phenomenon is significant in storage and packaging of chemicals because deliquescent substances require airtight containers to prevent unwanted liquefaction.